The present invention relates to a method for producing conductive particles.
Techniques for forming a conductive coating on a particle are drawing attention in various fields such as anisotropic conductive particles used for bonding a semiconductor chip or a liquid crystal panel, conductive paint for shielding electromagnetic wave, and electrode materials for secondary batteries of a great power.
In the case where a nonconductive material or a low conductive material is subjected to electroless plating, pretreatment process including immersion degreasing, surface conditioning, etching and catalyzing is usually required before electroless plating.
Commonly used catalyzing methods are a method in which a sensitizing step using stannous chloride and hydrochloric acid and an activation step using palladium chloride and hydrochloric acid are repeatedly (several times) performed with a water rinse between the sensitizing and activation steps, and a method in which a catalyzing step using a colloidal material made of tin and palladium and an activating step using hydrochloric acid or sulfuric acid to remove tin are repeatedly (several times) performed with a water rinse between the catalyzing and activation steps. These methods are disclosed, for example, in Japanese Patent Laid-Open No. 2000 -294236.
Both these methods are based on a mechanism in which a catalyst is activated by reducing palladium ions into metallic palladium in the presence of divalent tin ions with reducing power. The catalyzing step as described above may sometimes be used in the case of forming a metal plate coating with a lower potential on a metal with a higher potential such as the case where the surface of copper is plated with nickel.
Conductive particles can be produced by the electroless plating as described above, but the conventional methods are complicated because of the pretreatment process. Moreover, in order to prevent particles from carrying a solution obtained at one step into the next step, the conventional methods require, for example, repeated operations of rinsing and filtration, which requires a large amount of time.
Particles used for producing conductive particles have a very large specific surface area of about 0.2 to 0.5 m2/g. For example, 100 g of particles have a total surface area of 5000 DCM2 at the maximum, which is much larger than the area that is normally plated. Because of this, in the conventional catalyzing step, the amount of palladium to be adsorbed becomes insufficient, which makes it difficult to form an electroless plate coating. In addition to this, the use of a large amount of treatment solution in consideration of the bath ratio creates a great cost burden because costly catalyzer is spent in only one step.
In view of the above, it is an object of the present invention to provide a low-cost method for producing conductive particles in a short period of time by simplifying pretreatment process in the electroless plating.